Strifeshadow tells me that it doesn't have the necessary files to play the game.

There are three causes of this problem presented here in order of decreasing liklihood:

Some other application has locked one or more of your Strifeshadow file. This can happen if Strifeshadow fails to shutdown. In this case, rebooting will solve the problem.

The Strifeshadow Demo is corrupted, usually as a result of an install error or some form of HD corruption. Use Scandisc or another similar utility to check for and correct this problem

(unlikely) In at least one circumstance, an overclocked machine was miscomputing the file ID information. If you have an overclocked processor, try turning down the speed temporarily to see if the issue dissapears.